1. Field of the Invention
This invention relates generally to force transducers and more particularly, to resistive strain gauge force transducers for actuators.
2. Prior Art
Force transducers convert external forces applied to an actuator by a user into corresponding electrical signals that are used to control a device or position an object of a display. The external forces are converted into electrical signals that represent force components along orthogonal axes. For example, a joystick-type display controller for a computer includes a force transducer that converts forces applied to the joystick into two signals, one signal representing the component of force along an X axis and a second signal representing the component of force along a Y axis that is orthogonal to the X axis. The relative magnitude of the X and Y signals represent the relative amount of display pointer movement along the X and Y axis desired by the user. A computer to which the display controller is attached receives the electrical signals and moves the display pointer accordingly. The force transducer is provided with electrical power to produce electrical signals representing the force components as the external force is applied.
The joystick-type display controller described above can include a force transducer comprising a lever arm that is mechanically coupled to a support base by an articulated joint. While the force transducer provides the needed force component electrical signals, the mechanical linkage needed to permit movement of the joystick in the desired directions is complex. In particular, the mechanical linkage is relatively large, bulky and expensive. Also, the mechanical linkage is subject to wear and reliability problems.
As an alternative to the joystick force transducer with a mechanical linkage, force transducers have been developed with thin film resistive strain gauges. A thin film resistive strain gauge uses a conductive, thin film resistive strain gauge material that is deposited onto a thin, flexible substrate using photolithographic fabrication techniques. The strain gauge material undergoes a change in electrical resistivity when the underlying substrate is subjected to strain. If an electrical current is passed through the strain gauge material, the change in resistivity can be detected by an output voltage change and the relative amount of strain can be measured. The substrate is bonded onto the sides of the lever arm so that the substrate undergoes strain when forces are applied to the lever arm.
For example, some laptop computers are provided with a display controller force transducer comprising a pointer actuator that extends upwardly from between keys of the computer keyboard. The lever arm includes the thin film strain gauges described above. As forces are applied to the lever arm, the strain is detected and is used to control a display pointer. This permits a user to control the associated display pointer without removing his or her hands from the keyboard. The user's hands otherwise would need to move from the keyboard to manipulate, for example, a display mouse controller or joystick controller.
The force transducer using thin film resistive strain gauge technology represents a valuable advance over mechanical linkage force transducers. The thin film force transducer is much smaller and lighter than the mechanical linkage and therefore is ideally suited to installation in tight spaces, such as beneath the keyboard of a laptop computer. Moreover, the thin film force transducer contains no moving parts. Thus, the wear and reliability problems of the mechanical linkage are virtually eliminated. Unfortunately, the thin film resistive strain gauges produce output signals with a relatively low signal-to-noise ratio. The electrical environment within a laptop computer includes many different sources of electrical and magnetic interference that make processing the force transducer output signals difficult. In addition, the thin film resistive strain gauges are relatively expensive to produce.
Therefrom there is a need for a force transducer that is sufficiently small to be suitable for use in tight spaces such as computer keyboards, which provides improved signal-to-noise characteristics for easier signal processing, is reliable for improved performance and greater service life, and can be produced at a competitive cost.
In a prior invention of which the present applicant is a co-inventor, a force transducer includes an elongated lever arm or post attached to a substrate having strain gauges constructed from thick film resistive strain gauge material. The thick film strain gauge material is deposited directly onto the substrate using thick film screen printing techniques, which are much less expensive than thin film photolithographic techniques. The thick film strain gauges provide a force transducer that is much smaller than a mechanical transducer, has better reliability, can be produced for less cost than thin film force transducers, and which provides output signals with higher signal-to-noise ratios.
The post has a fixed end and a free end to which an external force is applied. The substrate includes a central portion at which the fixed end of the post is attached and substantial planar tab regions that project outwardly from the central portion along first and second orthogonal force-detecting axes so that the substrate undergoes localized strain approximately at the junctions of the tab regions and the central portion when the external force is applied to the free end of the post. As noted above, the thick film strain gauge material is printed directly onto the substrate. Such monolithic construction provides a force transducer that is much easier to produce than the thin film photolithographic technology commonly used.
In one aspect of that prior invention, the thick film resistive strain gauge material is printed in first and second locations such that the strain gauge material at each location bridges the central portion and a tab region across a respective tab region-central portion junction of localized strain. The force transducer includes conductive pads on the substrate that are electrically coupled to the thick film strain gauge material at each location to define a first strain gauge having an electrical path through the first location oriented along the first force detecting axis and a second strain gauge having an electrical path through the second location oriented along the second force detecting axis.
In that prior invention, the substrate may comprise a substantially planar material having a top surface and a bottom surface, and the strain gauge material may be deposited on both the top surface and bottom surface. Combining the electrical from the top strain gauges and the bottom strain gauges cancels out temperature effects and improves the signal-to-noise ratio of the output signal.
Although the aforementioned prior invention solves many of the noted deficiencies of the prior art, there are certain additional features which would be highly advantageous but which are simply not found in the prior invention. For example, it would be very useful to provide a capability to measure force in a third orthogonal direction such as in a vertical direction. Furthermore, it would be useful to provide more convenient structural implementations such as easier methods of integrating a sensor and a support plate to produce a transducer. In addition, given the tendency of a user to apply extremely high forces to a joystick controller or of a robot to apply high forces through a robotic position or shape sensor, there would be advantages to protecting the transducer from being overly stressed. Furthermore, it would be highly advantageous to provide a fabrication scheme which made the gauges less sensitive to surface irregularities which may be incurred during installation and which otherwise render transducer parameters less repeatable from unit to unit. It will be seen hereinafter that the present invention provides all of these additional capabilities.